Reliability Performance of Palladium Coated Copper Wire in High Temperature Bake at Extreme Durations

Abstract

New automotive requirements expect plastic packages to survive higher operating temperatures with extended thermal duration. Further module integration and more stringent environmental requirements push modules and thus plastic packages closer to the heat source. As such, new mission profiles include more than 3500 total hours at 150°C, or 1000 total hours at 175°C. To satisfy new automotive requirements, plastic packages must meet AEC Grade 0 or higher. Therefore, there is a need to evaluate the wire bond interconnect reliability to 2X AEC Grade 0 to assess the interconnect robustness. Palladium coated copper wire (Pd-Cu) was selected to be the wire of choice on aluminum bond pad for high reliability. Low Profile Quad Flat Pack with Exposed Pad (LQFP-EP) packages wire bonded with 20 micron Pd-Cu wire were submitted to high temperature baking (HTB) at 150°C and 175°C for 4032 and 2016 hours, respectively. This paper will discuss the results of electrical test, package deprocessing and wire pull testing, and cross-sectioning through the ball bonds to examine the welding region. Cu voiding within the bulk of the Cu ball bond, specifically close to the capillary chamfer squeezed region on top of the squashed ball, and at the ball edge interfacing with the intermetallic compound was found predominantly at 175°C bake. A trace of Sulfur was found inside the Cu corroding region. Sulfur compounds in mold compounds were examined. EFO current settings were evaluated in order to reduce the propensity to Sulfur corrosion in the Pd-Cu bond balls. Cu voiding is considered as cosmetic defect as it did not lead to electrical failures even at 2X AEC Grade 0 durations. Pd-Cu wire on aluminum bond pad forms very robust and reliable interconnect in extended thermal aging condition, including 4032 hour HTB-150°C and 2016 hour HTB-175°C, making it suitable for more demanding automotive mission profiles.